Optical transmission systems built around optical fibers have become widely used for broadband communication of digital data. Differential phase shift keying (DPSK) is a modulation technique that is widely used for transforming the digital data into optical pulses carried by the optical fibers. DPSK's coding scheme assigns phase shifts between adjacent symbols to numbers (zero and one, since digital data is predominantly binary). Timeslots are defined, and an optical pulse is generated during each timeslot. The optical pulse in each timeslot is phase-shifted from the optical pulse in the preceding timeslot as a function of the coding scheme to transmit the digital data from a transmitter to a receiver. Binary DPSK (DBPSK) carries one bit per optical pulse. Quadrature DPSK (DQPSK) expands the bandwidth binary DPSK offers by dividing each optical pulse into DPSK-modulated in-phase and quadrature components, allowing two bits to be carried per optical pulse. DPSK can encompass still higher constellation transmission formats and therefore higher numbers of bits per pulse.
Conventional optical DQPSK receivers (an optical processor that receives pulsetrains) demodulate the received DQPSK-modulated optical pulses by first dividing the pulsetrain into two identical components labeled in-phase (I) and quadrature (Q). Then, for each of the I and Q pulse components, predecessor and successor pulse components are made to interfere with each other. Each interference reveals the phase-shift in each component and thus the digital data contained in the optical pulse.
To perform these functions, conventional optical DQPSK receivers first use a splitter to divide the optical components into I and Q. Then, two separate interferometers cause the predecessor and successor I pulse components to interfere with each other and the predecessor and successor Q pulse components to interfere with each other. Finally, a differential receiver pair associated with each of the interferometers detects the resulting interference of both the I and Q pulse components. One example of a conventional optical DQPSK receiver is described in Griffin, et al., “Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission,” OFC 2002, WX6, Anaheim, Calif., USA, March 2002, incorporated herein by reference.
Each interferometer in the optical DQPSK receiver uses a delay line to delay predecessor pulses. The delay in each delay line must be adjusted continually to ensure a proper coincidence of the predecessor and successor pulses. A heating unit performs this function. Each of the two delay lines requires its own heating unit. Thus, a conventional DQPSK receiver requires two heating units. Unfortunately, heating units (and their associated thermostat control circuitry) require electric power, which is why such delay lines are called “active.” Heating units in fact consume significant electric power.
The two interferometers must also be polarization-independent. That is, the propagation velocity of the pulses through the interferometers should not depend upon their polarization. Unfortunately, it is difficult and therefore expensive to produce a receiver having a pair of polarization-independent interferometers.
It is apparent that optical DQPSK receivers require several optical elements to function, which, like all optical devices, should be precisely aligned with each other to ensure proper reception. For this reason and to render the receiver as small as possible, conventional optical DQPSK receivers are typically formed in or on a “silicon optical bench,” or SiOB, which constitutes an optical substrate for the optical elements.
What is needed in the art is a design for an optical DQPSK pulsetrain processor that has fewer optical elements, especially active elements, and thus is capable of attaining smaller sizes and lower complexity than conventional optical DQPSK optical processors.